Electric typewriter/printer carriages are often driven by means of a motor acting through a system of two opposing cables operating in parallel, such as disclosed in U.S. Pat. No. 3,872,960. One end of each cable is anchored rigidly to the motor hub. The other end of a first of the cables is anchored rigidly to the typewriter/printer frame while the other end of the second of the cables is connected in series with a spring which tensions that cable to the typewriter/printer frame. The series spring arrangement is utilized to compensate for axial changes in length of the cables due to thermal, aging, and repeated high speed loading effects on the cables such that constant cable tension is maintained.
In cable drive systems, it is desired that the connections of the cables to the carriage and the motor hub appear mechanically stiff or rigid. A drawback of the described series spring arrangement is that the presence of the spring nearly completely eliminates the stiffness of the second cable in the connection of the carriage to the motor hub with consequent deterioration of carriage horizontal registration. A known arrangement which attempts to eliminate that drawback utilizes a non-rotating pulley around which the second cable is wrapped a fraction of a turn before reaching the spring. That arrangement is effective in restoring the apparent stiffness of the second cable but has the disadvantage of introducing a large uncertainty in the value of the tension applied by the second cable. Thus, there is a need for a spring biased cable tensioning arrangement which provides constancy in the value of the mean tension applied to the cables, and cable anchoring that appears perfectly rigid against high frequency load variations.